Asymmetric Structural Features in Single Supported Lipid Bilayers Containing Cholesterol and GM1 Resolved with Synchrotron X-Ray Reflectivity

AbstractThe cell membrane comprises numerous protein and lipid molecules capable of asymmetric organization between leaflets and liquid-liquid phase separation. We use single supported lipid bilayers (SLBs) to model cell membranes, and study how cholesterol and asymmetrically oriented ganglioside receptor GM1 affect membrane structure using synchrotron x-ray reflectivity. Using mixtures of cholesterol, sphingomyelin, and 1,2-dioleoyl-sn-glycero-3-phosphocholine, we characterize the structure of liquid-ordered and liquid-disordered SLBs in terms of acyl-chain density, headgroup size, and leaflet thickness. SLBs modeling the liquid-ordered phase are 10Å thicker and have a higher acyl-chain electron density (〈ρchain〉=0.33e−/Å3) compared to SLBs modeling the liquid-disordered phase, or pure phosphatidylcholine SLBs (〈ρchain〉=0.28e−/Å3). Incorporating GM1 into the distal bilayer leaflet results in membrane asymmetry and thickening of the leaflet of 4–9Å. The structural effect of GM1 is more complex in SLBs of cholesterol/sphingomyelin/1,2-dioleoyl-sn-glycero-3-phosphocholine, where the distal chains show a high electron density (〈ρchain〉=0.33e−/Å3) and the lipid diffusion constant is reduced by ∼50%, as measured by fluorescence microscopy. These results give quantitative information about the leaflet asymmetry and electron density changes induced by receptor molecules that penetrate a single lipid bilayer